Edwal Antistatic Film cleaner. You can get a bottle of it at any camera store. I bought a 4oz bottle for about $9. It is made for cleaning film, especially slides. I use it to get rid of fingerprints and other oily garbage on slides.

Use compressed air first to get rid of obvious dust. Then, apply the film cleaner to a wad of lens cleaning paper. Use the moist lens cleaning paper to clean the slide. Once the greasy spots are gone, a final brushing with an antistatic brush leaves your slides spotless.

Most of the active ingredients seem to be isopropyl alcohol. It does dry your fingers if you use it with bare skin touching the wad. Be wary of using it as a camera cleaner - it can strip off lettering if you're not careful.